Research On DAC Harmonic Predistortion Based On Adaptive FIR Filter

In many fields such as communications,radar,electronic countermeasures,and aerospace,frequency source is the core component in the systems and the fully digital direct digital frequency synthesis(DDS)technology is widely used,but the DDS spurious restrict its development.At present,the spurious sources other than DAC in DDS have corresponding measures,but there is no completely effective way to deal with the influence of DAC nonlinearity.A relatively feasible solution is to use the predistortion method to directly suppress the spurious spectral lines on the spectrum.However,the current predistortion methods have some shortcomings and have limitations in practical applications.Based on the idea of predistortion,this paper studies the predistortion of the harmonic spurious introduced by DAC,which aims to overcome the shortcomings of traditional predistortion methods and improve the performance of DDS.Based on an in-depth analysis of the nonlinear effects and actual performance of DAC,this paper summarizes the characteristics of DAC output harmonics,analyzes them using mathematical expressions,and proposes that FIR filters can be used to fit harmonic frequency domain changes,studied the establishment of harmonic model through theoretical derivation.In terms of model solution and design,based on the improved RLS algorithm,this paper proposes an adaptive FIR filter frequency domain design method,that is,multi-frequency learning structure.The proposed design method can facilitate the design of special frequency response filters,and can adjust the frequency domain after the design is completed,which has the design capabilities and advantages that traditional filter design methods do not have.Based on the harmonic model and adaptive filter design method,this paper designs a new harmonic predistortion system,which does not require reconfiguration for different DDS output frequencies,greatly improving the applicability of predistortion method in DDS applications.Tests in the MATLAB platform show that the system design of this paper achieves real-time harmonic predistortion in the entire set frequency band,the SFDR of the output spectrum is improved by up to 26.7d B,and the THD is improved by up to 28.3d B,the purity of the spectrum in the entire frequency band has been improved by at least 10 dB.